bpf: Fix struct_meta lookup for bpf_obj_free_fields kfunc call

bpf_obj_drop_impl has a void return type. In check_kfunc_call, the "else
if" which sets insn_aux->kptr_struct_meta for bpf_obj_drop_impl is
surrounded by a larger if statement which checks btf_type_is_ptr. As a
result:

  * The bpf_obj_drop_impl-specific code will never execute
  * The btf_struct_meta input to bpf_obj_drop is always NULL
  * __bpf_obj_drop_impl will always see a NULL btf_record when called
    from BPF program, and won't call bpf_obj_free_fields
  * program-allocated kptrs which have fields that should be cleaned up
    by bpf_obj_free_fields may instead leak resources

This patch adds a btf_type_is_void branch to the larger if and moves
special handling for bpf_obj_drop_impl there, fixing the issue.

Fixes: ac9f0605 ("bpf: Introduce bpf_obj_drop")
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230403200027.2271029-1-davemarchevsky@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -10743,10 +10743,6 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 				insn_aux->obj_new_size = ret_t->size;
 				insn_aux->kptr_struct_meta =
 					btf_find_struct_meta(ret_btf, ret_btf_id);
-			} else if (meta.func_id == special_kfunc_list[KF_bpf_obj_drop_impl]) {
-				insn_aux->kptr_struct_meta =
-					btf_find_struct_meta(meta.arg_obj_drop.btf,
-							     meta.arg_obj_drop.btf_id);
 			} else if (meta.func_id == special_kfunc_list[KF_bpf_list_pop_front] ||
 				   meta.func_id == special_kfunc_list[KF_bpf_list_pop_back]) {
 				struct btf_field *field = meta.arg_list_head.field;
@@ -10875,7 +10871,15 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 
 		if (reg_may_point_to_spin_lock(&regs[BPF_REG_0]) && !regs[BPF_REG_0].id)
 			regs[BPF_REG_0].id = ++env->id_gen;
-	} /* else { add_kfunc_call() ensures it is btf_type_is_void(t) } */
+	} else if (btf_type_is_void(t)) {
+		if (meta.btf == btf_vmlinux && btf_id_set_contains(&special_kfunc_set, meta.func_id)) {
+			if (meta.func_id == special_kfunc_list[KF_bpf_obj_drop_impl]) {
+				insn_aux->kptr_struct_meta =
+					btf_find_struct_meta(meta.arg_obj_drop.btf,
+							     meta.arg_obj_drop.btf_id);
+			}
+		}
+	}
 
 	nargs = btf_type_vlen(meta.func_proto);
 	args = (const struct btf_param *)(meta.func_proto + 1);